1. Field of the Invention
The present invention relates to a fluid state detection apparatus for detecting the state of a fluid within a target atmosphere.
2. Description of the Related Art
Fluid state detection apparatuses have been used to detect the state of a fluid (concentration of gas, flow rate of gas, flow rate of liquid, temperature, or the like) within a target atmosphere.
A known fluid state detection apparatus for detecting the state of a fluid within a target atmosphere includes a heat generation resistor whose resistance changes with the fluid state to be detected. This fluid state detection apparatus measures, for example, the amount of heat transferred from the heat generation resistor to the fluid to be detected and calculates a change in the heat conduction of the fluid to be detected. In this manner, the fluid state detection apparatus can detect the fluid state (for example, hydrogen concentration, etc.).
A fluid state detection apparatus has been known which is configured to measure the amount of heat transferred from a heat generation resistor to a fluid to be detected using a Wheatstone bridge circuit (hereinafter also referred to as a “bridge circuit”) which includes the heat generation resistor as one of four resistor sections.
In the case of a fluid state detection apparatus in which such a bridge circuit is used, an open failure may occur in which the connection between the bridge circuit and the inverting input terminal of an operational amplifier is broken, or an open failure in which the connection to the output terminal of the operational amplifier is broken.
In view of the above, a fluid state detection apparatus which detects such an open failure has been proposed (Patent Document 1). In the proposed fluid state detection apparatus, the resistance of each of the individual resistor sections of the bridge circuit is properly set such that when the above-mentioned open failure occurs, the output of the operational amplifier deviates from its normal output range, whereby the fluid state detection apparatus can detect the open failure.
Incidentally, in recent years, in consideration of environmental protection and nature conservation among other societal demands, research has been actively conducted on fuel cells, which are energy sources of high efficiency and low environmental load. Among various types of fuel cells, a polymer electrolyte fuel cell (PEFC) has drawn attention as an energy source for home use or an energy source for vehicles because of its advantageous low operation temperature and high output density. Such a polymer electrolyte fuel cell uses, as a fuel, hydrogen which is more likely to leak as compared with other fuels. Therefore, a fluid state detection apparatus which detects hydrogen leakage has become necessary for practical implementation of a polymer electrolyte fuel cell.
Also, research has been actively conducted on a hydrogen internal combustion engine which is an energy source having a low environmental load and which uses hydrogen as a fuel similar to the polymer electrolyte fuel cell. As for the hydrogen internal combustion engine as well, a fluid state detection apparatus which detects hydrogen leakage has become necessary for practical implementation.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2012-198093
3. Problem to be Solved by the Invention
However, in the case of the above-described fluid state detection apparatus, when the bridge circuit is shorted to a power supply (for example, when a short failure has occurred in an energization control section which controls the supply of current from the power supply to the bridge circuit), detection of the short failure is difficult.
Namely, since the output of the operational amplifier at the time of the short failure (in which the bridge circuit is shorted to the power supply) remains within the normal output range of the operational amplifier, it is difficult to detect such a failure by the above-described method of properly setting the resistances of the resistor sections of the bridge circuit as described above.
For example, in the case where the resistance of the heat generation resistor decreases due to the fluid state to be detected, as a result of the feedback control performed by the operational amplifier, the current supplied from the energization control section to the bridge circuit is controlled to a maximum value (or the applied voltage is controlled to a maximum value). This is a control state created as a result of detecting the fluid state, and is a normal control state of the fluid state detection apparatus.
Meanwhile, when the energization control section (for example, a transistor) enters a short failure state and the bridge circuit is shorted to the power supply, the supply of current from the power supply to the bridge circuit cannot be controlled, whereby the current supplied to the bridge circuit assumes a maximum value (or the applied voltage assumes a maximum value). This is a state created as a result of a short failure of the energization control section, and is an anomaly state (failure state) of the fluid state detection apparatus.
As described above, the current supplied to the bridge circuit becomes a maximum (or the applied voltage becomes a maximum) and the output of the operational amplifier assumes the same value in both the normal state in which the resistance of the heat generation resistor decreases and a short failure state of the energization control section (in other words, when the bridge circuit is shorted to the power supply). Therefore, it is difficult to distinguish the two states from each other based on the output of the operational amplifier.